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08-16-2016 11:55 AM

Hi,

I want to run a random vibration analysis for an assembly in Femap, but before performing the analysis, I have question about the modal effective mass. Basically I try to understand more how are modes selected for random vibration. I found some online articles that say FEA software select modes to have at least 80% modal effective mass of the whole system. Do we need to always follow the 80% rule? Or can we just manually select the first few modes ? I hope my question is clear.

Thanks

2 REPLIES

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08-16-2016 05:53 PM

Hello!,

To perform any dynamic response analysis is required to compute as many modes as needed to account for minimum the 80% of the total mass of the structure in the direction of the excitation. The following picture shows in FEMAP the modal mass participation factor of the equipment vs. number of mode, we can see that extracting the 200 first modes we are well above of the 90%. The 80% is the minimum required, but 90% is better!.

Another good recomendation is to use enough modes to cover the range of excitation. For example, if the model is to be excited from 1 to 100 Hz, you must use all of the modes with frequencies up to at least 100 Hz. This is only a minimum requirement, however. A better guideline is to use enough modes to cover two to three times the range of excitation in order to provide accurate answers at the high end of the frequency range. For example, when excitation is applied to 100 Hz, modes with frequencies up to 200 to 300 Hz should all be used.

Best regards,

Blas.

Blas Molero Hidalgo, Ingeniero Industrial, Director

IBERISA • 48004 BILBAO (SPAIN)

WEB: http://www.iberisa.com

Blog Femap-NX Nastran: http://iberisa.wordpress.com/

IBERISA • 48004 BILBAO (SPAIN)

WEB: http://www.iberisa.com

Blog Femap-NX Nastran: http://iberisa.wordpress.com/

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08-17-2016 09:58 AM - edited 08-19-2016 11:04 AM

Thanks Blas for your reply. I could not see your attached picture for some reasons, but I got the idea.

I just have a question to clarify my doubts. If my interested frequency is between 20 and 2000 Hz, then I select the 3 times the modes in the frequency range, which turns out to be 10000 modes. But these 10000 modes do not add up to 80% of the modal effective mass. Should I keep add more modes until it reaches 80%? Or these 10000 modes should be sufficient because they are already within and beyond my interested frequency range.

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